As an environmental pollution problem has emerged as a major social issue, exhaust gas regulations for internal-combustion engine automobiles using fossil fuel has become tightened. In particular, an exhaust gas emitted from automobiles using diesel fuel contains nitrogen oxides (NOx). Since nitrogen oxides may cause acid rain or severe respiratory problems, tough emission standards of nitrogen oxides included in exhaust gases are applied in all over the world.
A selective catalyst reduction (SCR) method and an exhaust gas recirculation (EGR) apparatus are widely used to remove nitrogen oxides contained in exhaust gases. In the SCR method, a nitrogen oxide in an exhaust gas is removed by reducing the nitrogen oxide to nitrogen and water using urea water as a catalyst.
Since the SCR method uses urea water as a catalyst, a tank for storing urea water and a device for supplying urea water stored in the tank to an exhaust gas are required in an automobile. The amount of urea water stored in the tank is about 5 to 7% of the amount of fuel. Urea water stored in the tank continues to be used to remove nitrogen oxides so urea water needs to be periodically charged, and thus, in order to measure a remaining amount of urea water, a device which may be installed in the tank to measure a level of urea water is required.
Conventionally, there are two methods of measuring a level of urea water: One is a method of measuring electric conductivity varied according to levels of urea water and the other is a method of using an ultrasonic wave. The related art method using electric conductivity is disclosed in Korean Patent Registration No. 10-1584634 (“Urea water level measuring apparatus” published on Jan. 6, 2016, referred to as Related Art 1 hereinafter) and the related art method using ultrasonic waves is disclosed in Korean Patent Registration No. 10-1323399 (“Urea water level measuring apparatus using ultrasonic level sensor”, published on Oct. 23, 2013, referred to as Related Art 2 hereinafter).
The method of measuring electric conductivity, that is, inductance, disclosed in Related Art 1 has a problem in that a measurement value is inaccurate when urea water is changed into bubbles, and the ultrasonic method used in Related Art 2 has a problem in that a measured level of urea water is inaccurate when urea water is exposed to low temperatures and freezes.